1. Field of the Invention
This invention relates to carriers for mounting magnetic heads to a drum section of a helical scan magnetic tape unit.
2. Description Relative to the Prior Art
Accurate positioning of magnetic heads with respect to a rotatable head drum element is essential for proper operation of most types of magnetic tape scanning units. Such head alignment requirements, moreover, tend to become more exacting as the recorded information density increases. With helical tape scanning, as is commonly employed in video recording, magnetic heads generally cooperate in pairs arranged on opposite sides of a drum section, and such use of cooperating head pairs further complicates the problems of head alignment by introducing critical head pair alignment geometries.
Usually, it is desired that the heads of a head pair trace a common circular path as they are rotated. To satisfy such a requirement, the heads of a cooperating pair must be aligned to rotate in a single plane (coplanar alignment) and, for units characterized by a helical tape wrap path, a plane normal to the head drum axis is almost always chosen. In the absence of coplanar alignment for the head pair under discussion, the tracks produced by a helical scanner depart from the desired uniform spacing and, instead, alternate between wide and narrow spacing. Such spacing variation, if significant in relation to track width, may frustrate proper tracking of the recorded information. Either the recording or the playback scanner, it should be appreciated, may be subject to coplanarity error and, hence, cause such a tracking failure.
Further important alignment geometries relate to the arrangement of the heads within their plane of rotation and with respect to the head drum axis. To assure that the heads of a head pair trace the same circle, the radial distances of the heads from the head drum axis must be uniform and, moreover, to achieve a desired head response, the protrusion of the heads beyond the drum section periphery must be within a preselected tolerance.
An alignment geometry which is particularly critical relates to the positioning of the axis which passes through the heads of a head pair (head pair axis) with respect to the rotary head drum axis. Such alignment is commonly referred to as dihedral alignment. Generally, the head pair axis must intersect the rotary head drum axis (some scanners do, however, incorporate a fixed angular offset -- see, for example, U.S. Pat. No. 3,391,248). If such axes do not intersect, i.e., if there is dihedral error, the start points of scan tracks will alternate in distance from the tape edge and, if the unit is used for video scanning, picture jitter is a likely consequence of such offsetting of tracks.
To deal with the complex geometries of head alignment, various head mounting arrangements have been proposed.
One approach calls for direct bonding of the heads to a rotatable drum section. This approach generally results in an expensive head carrying element and does not lend itself to economical head replacement. Furthermore, no adjustment after bonding is possible and all of the alignments, both for the individual heads and for cooperating head pairs, must be accomplished in one operation.
To permit adjustment flexibility and ease of head replacement, a variety of head carriers have been proposed which allow for adjustments to the arrangement of the individual heads. Certain of these allow for adjustment along preselected paths; for example, along a chord of a rotating drum section (U.S. Pat. No. 3,679,838). Others provide several adjustments for positioning individual heads; for example, U.S. Pat. Nos. 3,319,015 and 3,882,543.
In U.S. Pat. No. 3,422,230, there is described a rectangular strap which acts as a carrier for a pair of heads. The strap is adapted to be mounted to a scanning disc of the head drum. All head and head pair alignment is intended to be performed prior to attachment of the strap to the scanning disc. While such prior alignment is somewhat advantageous, it requires rather exact mating of the strap to the scanning disc and exact positioning of the mating surfaces relative to the scanning disc axis. Should one alignment, say dihedral, prove to be incorrect, then apparently the heads must be individually repositioned to remove the alignment error.